A description of a typical laser powder bed fusion process is provided in German Patent No. DE 19649865, which is incorporated herein by reference in its entirety. AM processes generally involve the buildup of one or more materials to make a net or near net shape (NNS) object, in contrast to subtractive manufacturing methods. Though “additive manufacturing” is an industry standard term (ASTM F2792), AM encompasses various manufacturing and prototyping techniques known under a variety of names, including freeform fabrication, 3D printing, rapid prototyping/tooling, etc. AM techniques are capable of fabricating complex components from a wide variety of materials. Generally, a freestanding object can be fabricated from a computer aided design (CAD) model. A particular type of AM process uses an irradiation emission directing device that directs an energy beam, for example, an electron beam or a laser beam, to sinter or melt a powder material, creating a solid three-dimensional object in which particles of the powder material are bonded together. Different material systems, for example, engineering plastics, thermoplastic elastomers, metals, and ceramics are in use. Laser sintering or melting is a notable AM process for rapid fabrication of functional prototypes and tools. Applications include direct manufacturing of complex workpieces, patterns for investment casting, metal molds for injection molding and die casting, and molds and cores for sand casting. Fabrication of prototype objects to enhance communication and testing of concepts during the design cycle are other common usages of AM processes.
Selective laser sintering, direct laser sintering, selective laser melting, and direct laser melting are common industry terms used to refer to producing three-dimensional (3D) objects by using a laser beam to sinter or melt a fine powder. For example, U.S. Pat. No. 4,863,538 and U.S. Pat. No. 5,460,758, which are incorporated herein by reference, describe conventional laser sintering techniques. More accurately, sintering entails fusing (agglomerating) particles of a powder at a temperature below the melting point of the powder material, whereas melting entails fully melting particles of a powder to form a solid homogeneous mass. The physical processes associated with laser sintering or laser melting include heat transfer to a powder material and then either sintering or melting the powder material. Although the laser sintering and melting processes can be applied to a broad range of powder materials, the scientific and technical aspects of the production route, for example, sintering or melting rate and the effects of processing parameters on the microstructural evolution during the layer manufacturing process have not been well understood. This method of fabrication is accompanied by multiple modes of heat, mass and momentum transfer, and chemical reactions that make the process very complex.
FIG. 1 is schematic diagram showing a cross-sectional view of an exemplary conventional system 100 for direct metal laser sintering (“DMLS”) or direct metal laser melting (DMLM). The apparatus 100 builds objects, for example, the part 122, in a layer-by-layer manner by sintering or melting a powder material (not shown) using an energy beam 136 generated by a source 120, which can be, for example, a laser for producing a laser beam, or a filament that emits electrons when a current flows through it. The powder to be melted by the energy beam is supplied by reservoir 126 and spread evenly over a powder bed 112 using a recoater arm 116 travelling in direction 134 to maintain the powder at a level 118 and remove excess powder material extending above the powder level 118 to waste container 128. The energy beam 136 sinters or melts a cross sectional layer of the object being built under control of an irradiation emission directing device, such as a galvo scanner 132. The galvo scanner 132 may comprise, for example, a plurality of movable mirrors or scanning lenses. The speed at which the laser is scanned is a critical controllable process parameter, impacting how long the laser power is applied to a particular spot. Typical laser scan speeds are on the order of 10 to 100 millimeters per second. The build platform 114 is lowered and another layer of powder is spread over the powder bed and object being built, followed by successive melting/sintering of the powder by the laser 120. The powder layer is typically, for example, 10 to 100 microns. The process is repeated until the part 122 is completely built up from the melted/sintered powder material.
The laser 120 may be controlled by a computer system including a processor and a memory. The computer system may determine a scan pattern for each layer and control laser 120 to irradiate the powder material according to the scan pattern. After fabrication of the part 122 is complete, various post-processing procedures may be applied to the part 122. Post processing procedures include removal of excess powder by, for example, blowing or vacuuming. Other post processing procedures include a stress release process. Additionally, thermal and chemical post processing procedures can be used to finish the part 122.
FIG. 2 shows a side view of an object 201 built in a conventional powder bed 202, which could be for example a powder bed as illustrated by element 112 of FIG. 1. Then as the build platform 114 is lowered and successive layers of powder are built up, the object 201 is formed in the bed 202. The walls 203 of the powder bed 202 define the amount of powder needed to make a part. The weight of the powder within the build environment is one limitation on the size of parts being built in this type of apparatus. The amount of powder needed to make a large part may exceed the limits of the build platform 114 or make it difficult to control the lowering of the build platform by precise steps which is needed to make highly uniform additive layers in the object being built.
In conventional powder bed systems, such as shown in FIG. 1, the energy beam 136 must scan a relatively large angle ⊖1 when building a part large enough to occupy most of the powder bed 118. This is because the angle ⊖1 must increase as the cross-sectional area of the object increases. In general, when making these larger parts, the angle ⊖1 becomes large at the periphery of the part. The energy density at the point of contact between the laser and powder bed then varies over the part. These differences in energy density affect the melt pool at large angles relative to that obtained when the laser is normal to the powder bed. These melt pool differences may result in defects and loss of fidelity in these regions of the part being built. These defects may result in inferior surface finishes on the desired part.
Another problem that arises with prior art methods and systems involves cooling the layer of powdered material and removing smoke, condensates, and other impurities produced by irradiating the powder (sometimes called the “gas plume”), which can contaminate the object and obscure the line of sight of the energy beam. It is also important to cool and solidify the layer quickly to avoid formation of deformations or other defects. For large objects, i.e. objects with a largest dimension in the xy plane (for conventional powder bed systems, the plane of the powder bed) of 400 to 450 mm, it is very difficult to provide consistent laminar gas flow and efficient removal of unwanted gasses, particulates, condensates, and other undesirable impurities and contaminants.
Another problem that arises in the prior art systems and methods is the need to finely control the quantity and location of powder deposited to avoid wasting powder, while also avoiding contact of the powder with undesirable materials. Prior art methods and systems deposit powder using blowing, sliding, or auger mechanisms. These mechanisms utilize multiple moving parts that may malfunction, or may be made of materials that are not suited to contact with the powder due to concerns with contamination.
For example, EP 2191922 and EP 2202016 to Cersten et al. discuss a powder application apparatus that dispenses powder using rotating conveyor shafts with recesses for holding separate, discrete amounts of powder. Such an apparatus is more prone to failure, because the rotating conveyor shafts must be in motion as long as powder is being deposited.
Other attempts to overcome the limitations of conventional powder bed systems have failed to address the problems associated with scale-up of these machines. In some cases, attempts to provide large format systems have introduced additional problems and challenges in creating laser fused parts from powder. Prior systems failed to provide uniform layer-wise powder distribution, effective management of the gas plume, and good control of the laser energy density over the part being produced.
For example, the concept of moving a laser within a build area was explored in U.S. Application Publication No. 2004/0094728 to Herzog et al., the present inventors have noted this disclosure does not address how powder might be distributed onto the part being built. These techniques imply more traditional laser powder deposition where powder is injected into a laser beam and melted onto the object being built. Because there is no discussion of how to achieve uniform layers or powder over the part being built, the dimensional accuracy of such systems are very limited. Moreover, because the build environment is large, achieving a suitable gas environment near the laser melt pool would be difficult.
In another example, the concept of a large format system whereby powder is deposited using a hopper is explored in U.S. Patent Application Publication No. 2013/0101746 to Keremes et al. Material 30 is deposited onto a part 40 being built using a material applicator 28. Retaining walls 42 are utilized to allow material 30 to build up as the part 40 is made. The system utilizes a laser 18 placed in a stationary position near the top of the build chamber. As the part 40 grows in size, the angle of the laser beam 20 increases, particularly at the peripheral regions of the part. In addition, because material 30 is deposited onto the part 40, the thickness of the material 30 deposited onto the part 40 is difficult to control precisely.
International Application No. WO 2014/199149 titled “Additive Manufacturing Apparatus and Method” to McMurtry et al. (“McMurtry”) discusses utilizing multiple polygonal mirrors with a localized gasflow device to build separate portions of an object in a single dimension, i.e. along a line, and lowering the build platform to provide another layer of powder. For large objects, it is difficult to build a platform that can both stably hold sufficient powder, and also be lowered by the precise layer thickness required.
There remains a need for a large format powder manufacturing system that overcomes the above-mentioned problems.